The barrier properties of alkanethiol-coated gold electrodes are dramatically enhanced when the surfactant 1-octanol is present in the aqueous electrolyte solution contacting the electrode. Gold oxidation on the electrode surface and ferrocyanide oxidation in solution are suppressed to a positive potential limit of +1.6 V vs Ag/AgCl in an octanol-saturated pH 5 buffer at a dodecanethiol-coated gold electrode. Hydroxymethylferrocene oxidation in solution is also suppressed, though to a lesser extent. The enhanced barrier properties are thought to be caused by a thin layer of octanol atop the alkanethiol monolayer. The octanol fills in defects in the alkanethiol monolayer and increases the overall thickness of the barrier layer, thereby inhibiting reactions at defects and forcing electron transfer to occur over long distances across the barrier layer. The beneficial effect of octanol is obtained only when the monolayer surface is hydrophobic (e.g., at an alkanethiol monolayer) and is absent when the monolayer surface is hydrophilic and/or charged (e.g., at a mercaptoundecanoic acid monolayer in a basic buffer solution).